An example of the conventional signal transmission system for a fire alarm junction line is shown in FIG. 1. In FIG. 1, on the receiving unit side, a junction line L.sub.11 is connected to a power source through a resistor R, and the collector electrode and the emitter electrode of a transistor T.sub.1 are respectively connected to the junction line L.sub.11 and another junction line L.sub.12 and a binary signal is applied to the base electrode of the transistor T.sub.1 so that a signal is sent out. On the repeater unit side, the sent-out signal is restored to its original state from a received waveform by distinguishing the potential of the junction line L.sub.11 with the aid of a comparator circuit A.sub.12. Another transistor T.sub.2 which is similar to the transistor T.sub.1 is provided on the repeater unit side and binary signals are applied to the base electrode of the transistor T.sub.2, thus a signal sent out from the repeater can be restored on the receiving unit side by means of another comparator circuit A.sub.11 which is similar to the comparator circuit A.sub.12. A plurality of fire detectors, not shown in the figure, are connected to the repeater and the code (number) of the fire detector that has issued a fire alarm signal is reported to the receiving unit by the repeater, while a signal for controlling smoke prevention and/or exhaust equipment can be sent to the repeater from the receiving unit side. As a plurality of repeaters are connected to a pair of junction lines, centralized administration of a plurality of fire detectors is possible with a junction line comprising a pair of wires.
However, when the number of repeaters connected to one pair of wires is increased and a plurality of fire detectors are connected to each repeater, signals must be sent out and received at a high speed through the junction line in order to transmit more detailed information. In the circuit as described above, a signal as shown in FIG. 2(a), which has been sent out is received in the waveform as shown in FIG. 2(b). The reason for this is as follows: When the transistor T.sub.1 turns ON, the potential of the junction line L.sub.11 drops immediately to zero potential. However, when the transistor T.sub.1 turns OFF, it takes some time for the junction line L.sub.11, which has a certain static capacity, to be charged by the battery through the resistor R. Accordingly, the conventional signal transmission system is defective in that the signals sent out at a high speed cannot be appropriately received on the receiving unit side. If a resistor R with lower resistance is used in order to alleviate the above-mentioned defect, the voltage drop due to the resistance of the junction line L.sub.11 becomes larger during the ON state of the transistor T.sub.2 when a signal is sent from the repeater side, and distinction of signals on the receiving unit side becomes difficult. In addition, this difficulty will be aggravated owing to the difference in the length of the junction line L.sub.11 to each repeater, and in the worst case the signals cannot be distinguished on the receiving unit side. So employment of a resistor R.sub.1 of lower resistance is impractical.
The object of this invention is to provide a signal transmission system for a fire alarm junction line in which signals can be transmitted at a high speed between a fire alarm receiving unit and a plurality of repeaters, in which the defects of the prior art systems are diminished.